In previous studies we identified a pneumococcal surface protein (PspA) which acts as a virulence factor for pneumococci and can elicit antibodies that are able to protect mice from fatal pneumococcal infection. The presence of PspA on pneumococci slows their clearance from the blood. While it seems likely that PspA interferes with opsonophagocytosis, the mechanism of its action is not known. The N-terminal half (49%) of PspA from strain Rx1 is a highly charged alpha-helical sequence with a strong coiled-coil motif that is followed by a proline-rich central region, and a repeat region which serves to anchor PspA to the bacterial membrane. PspA lacks a sequence consistent with a transmembrane anchor and the LPXTGE sequence implicated in surface attachment of most previously described gram positive surface proteins. All protective monoclonal antibodies (MAb) to PspA react with its N-terminal half and immunization with this portion of the molecule can elicit protection against fatal pneumococcal infection. The N-terminal half of PspA exhibits a high degree of serologic variability. In spite of this variability, the N-terminal region of each PspA contains epitopes cross-reactive with those of many other PspA molecules. Immune responses to one PspA have been shown to protect mice against fatal infection with strains bearing cross-reactive but serologically distinct PspA molecules. Recent evidence indicates that most strains have two sequences which strongly hybridize with cloned pspA. On a few strains two different PspA molecules have been demonstrated. We propose to identify the protection eliciting epitopes of PspA, determine the structural basis for the antigenic variability in PspA, investigate the mechanism of action of PspA, and learn the extent to which immunity to PspA might be able to protect against diverse strains of pneumococci. The data obtained will also allow us to test several hypotheses, including: 1) that the variability of PspA is the result of evolution to avoid protective antibodies, and 2) that PspA acts an anchor to hold the capsular polysaccharide to the pneumococcal surface, and 3) that most strains pneumococci may have two different PspA molecules.